The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Modern and future wireless communication systems are designed to provide high data rate capability. Accordingly, architectures of transmitters in wireless communication systems have evolved to accommodate complex modulation schemes with high spectral efficiency. These architectures are designed to ensure the transmission of wideband signals in a linear manner. Nonlinear behaviors in transmitters will distort the transmitted signal and hence corrupt the transmitted data. Radio frequency power amplifiers (RFPAs) are the most nonlinear and power consuming blocks in a transmitter's architecture. Therefore, special measures are taken to increase an RFPA linearity and efficiency.
Digital predistortion (DPD) is one of the most used linearization methods for cancelling out RFPAs nonlinear behavior. Predistortion is a technique used to improve the linearity of radio transmitter amplifiers. Predistortion circuit inversely models the amplifier's gain and phase characteristics and, when combined with the amplifier, produces an overall system that is more linear and reduces the distortions at the output of the amplifier. Specifically, predistortion circuit implements a nonlinear function upstream of the PA complementary to the nonlinear function of the PA to be linearized. Accordingly, the cascade made of the predistortion circuit and the PA will operates as a linear amplification system. Predistortion can be implemented in an analog as well as digital manner known as DPD.
DPD requires characterization of RFPAs. PA characterization process generates a behavior model describing nonlinear property of the PA. The behavior model can be a mathematical function including a set of coefficients. The coefficients can be calculated from PA input and output measurement data. Based on a PA behavior model, a DPD function inversing the PA behavior model can be created.
During the PA characterization process, PA output signal over a certain bandwidth is measured and analyzed. This bandwidth value is typically five times the input signal bandwidth. Therefore, in order to implement an effective DPD, the feedback path used to demodulate the PA output signal requires a baseband sampling rate several times faster than that of the baseband signal to be transmitted. For example, in modern wireless communication systems, the required sampling speed of the feedback path receiver is expected to be in the range of 500 Msps (Mega-samples per second) for 100 MHz wide LTE-Advanced signals.
In a research and development laboratory environment, power amplifiers can be characterized using modulated test signals. This requires the use of an arbitrary waveform generator (AWG) to feed the amplifier with the RF test signal and a vector signal analyzer (VSA) to demodulate the amplifier's output signal. Only expensive VSAs might be able to offer the measurement capabilities required by LTE and LTE-A signals. Moreover, in some cases, the available instruments cannot meet the sampling rate and dynamic range needed for PA linearization applications.
In addition to instrument based PA characterization and predistortion test benches, a wide range of other platforms was reported in the literature. For example, dedicated self-developed PA characterization and digital predistortion platforms using in-house designed signal generators and receivers were reported. In such type of platforms, the sampling rate limitation of the feedback path is also a serious challenge that needs to be addressed. Thus, it is essential to come up with experimental procedures that would allow for the extension of the observation bandwidth of vector signal analysis test and measurement equipment in particular, and the feedback path of experimental power amplifiers characterization and digital predistortion systems in general.
To address the above issue, several signal acquisition techniques have been reported in the literature for various applications. The Subsampling techniques were widely used in the presence of periodic signals with a relatively short time period value, as well as complex modulated signals. In the case of modulated signals, the use of under-sampling techniques requires spacing between the different carriers in order to avoid spectrum overlap. Accordingly, under-sampling techniques are successfully applied for dual band applications. However, under-sampling techniques are not suitable for wideband systems involving contiguous carrier aggregation. The frequency stitching technique provides wideband PA characterization by manipulating measurement data in the frequency domain leading to relatively complex signal processing.
Other approaches have been proposed to extend the bandwidth of digital predistortion systems. These include the band-limited Volterra series based digital predistortion approach in which the PA's and predistorter's outputs are filtered to reduce the sampling rate requirements. In this work, it was demonstrated that the band-limited Volterra series approach outperforms conventional DPD systems operating at the same sampling rate. Though, a major limitation is that the correction bandwidth of this technique is equal to its observation bandwidth in the sense that the band-limited DPD is unable to compensate for PA distortions outside the observation bandwidth. Digital predistortion architectures with a correction bandwidth that exceeds the observation bandwidth have been reported in another work. In this work, a spectral extrapolation technique was proposed to extend the correction bandwidth of DPD by means of computationally intensive signal processing algorithms. In a further work, an under-sampling restoration DPD (USR-DPD) was introduced. The USR-DPD uses an iterative approach to synthesize the predistortion function. Its ability to significantly reduce the required sampling rate in the feedback path was demonstrated. Though, to ensure distortion mitigation beyond the observation bandwidth, a band-pass filter is required. This can be perceived as a drawback since the analog band-pass filter characteristics are signal dependent. Thus, a different filter is required whenever the bandwidth of the input signal changes. In another work, a sequential two-step synthesis of a two-box digital predistorter was proposed. The proposed predistorter considerably reduces the required sampling rate in the feedback path. However, this technique is suitable for a specific type of digital predistortion functions which are built using a two-box structure made of the cascade of a first dynamic predistortion function followed by a memoryless one. Even though the above research results are of great interest to the case of LTE-A power amplifiers, there is a need for other alternative techniques for broadband behavioral modeling and digital predistortion using low speed analog to digital converters.